1. Field of the Invention
The invention in general relates to composite materials and particularly to an improved method for fabricating such materials.
2. Description of the Prior Art
A composite material, or structure, is one wherein a fiber system is embedded in a host material, also known as a matrix. The fiber system, which may be a woven cloth, individual parallel oriented or cross-plied fibers or randomly oriented short fibers or whiskers, by way of example, is introduced into the host material which is provided in liquid or powdered form. After combining, the mixture is subjected to further processing such as heat and/or pressure treatment resulting in the formation of a dense composite material.
The fibers, which are of high strength and stiffness, are utilized to reinforce the matrix which, in the case of metal matrices, results in increased creep and stress-rupture properties. In the case of ceramic matrices, the ceramic becomes less brittle, with greatly improved fracture toughness capabilities.
Such composite materials are used, and have potential uses in military as well as innumerable industrial applications such as in the building industry, chemical or other processing plants, air, surface and subsea vehicles, appliances, automotive parts, turbines and electrical components such as printed circuit boards, to name a few.
Various fiber-matrix combinations have resulted in composite materials which have failed to meet expectations, particularly in elevated temperature environments. The major cause of composite failure is an incompatibility between a particular fiber and a particular matrix, both from a chemical and mechanical standpoint. By way of example, at elevated operating temperatures, or at the temperatures needed to densify the material during fabrication of the composite, chemical reactions may occur between the fiber and the matrix which may actually corrode the surface of the fiber thereby reducing its strength by many orders or magnitude. In some cases, chemical reactions may also cause debonding between the fiber and matrix so as to prevent or inhibit the stress transfer mechanism which gives the composite its desired properties. In other cases, e.g., high toughness ceramic matrix/ceramic reinforced composites in which debonding is desirable, the chemical reactions may promote bonding. Problems also arise from a mismatch of thermal expansion coefficients between the fiber and host material.
To obviate these disadvantages, some composites are made by coating the fibers with an interface or barrier layer that is compatible with both the fiber and matrix so that the fiber properties are not degraded during processing or use. In addition, this barrier layer may be fabricated with a desired controlled expansion coefficient.
Presently, protective coatings, or barrier layers, are deposited on the fibers by techniques such as rf sputtering or vapor deposition. These processes, in addition to being relatively expensive, do not always uniformly coat the fibers such that uncoated or exposed portions will objectionably react with the host material.
Another method of barrier formation is by the addition of certain chemicals to the host material to hopefully react with the fiber to form a desired coating thereon. This technique requires ultra-precise control of the uniformity of the additives as well as precise control over processing temperature vs. time relationships.
In the composite material of the present invention, a protective barrier coating is formed on the fibers by a process which is simple, inexpensive and ensures for a controlled uniform coating on the fiber.